Project Summary/Abstract My goal is to quantitatively measure extracellular pH (pHe) in the tumor microenvironment to assess tumor acidosis. These assessments can be used to improve diagnoses of solid tumors, to aid in predicting the response to immunotherapy before the treatment is initiated, and to evaluate the early response of tumors to many types of drug treatment. These multiple applications can provide strong impact for studies of mouse tumor models, and eventually for patients who have solid tumors. To meet this goal, I propose to develop PET/MRI contrast agents that can quantitatively measure pHe, and apply these agents during simultaneous PET/MRI studies in mouse models of human cancers. Dynamic changes in the relaxation-based MR image contrast are sensitive to tumor pHe as well as the concentration of the agent in tumor tissue, while the PET image can be used to measure the concentration of the agent in the tumor. Therefore, the PET results can be used to account for the effect of concentration on MR image contrast, which can improve the quantitative measurement of tumor pHe. To overcome the difference in detection sensitivities of PET and MRI, I will co-inject 0.001% radiolabeled agent and 99.999% MRI contrast agent. Notably, this approach would fail to image cell receptors or intracellular biomarkers, but is ideally suited to interrogate the extracellular tumor microenvironment. In particular, these agents are designed to have the same pharmacokinetic delivery to the extracellular tumor microenvironment during the first 10 minutes after co- injection, so that the MBq radioactivity measurement with PET can be used to evaluate the M concentration of the MRI contrast agent. Simultaneous imaging of these agents will be performed using one of the few commercial PET/MRI systems for small animal imaging world-wide. Therefore, my development of contrast agents for simultaneous PET/MRI has strong innovation for cancer imaging. Accurate and precise measurements of tumor pHe requires great attention to rigor, especially to address potential inaccuracies and imprecisions with both imaging modalities. Therefore, I have designed a strong research approach with careful attention to rigor. A major component to this project is the design and synthesis of the non-radiolabeled and radiolabeled versions of the agent. I have improved upon the original synthesis of the pH responsive MRI agent to obtain an overall yield of 54%, and I have proposed a synthesis for the final fluorine-19 MRI component of the contrast agent. In addition, two novel fluorine-18 agents have been designed as possible radioactive counterparts. Ultimately, the PET/MRI co-agents will be chosen based on their pH responsiveness and their dynamic range for pH detection. My deliverable is a fundamentally new class of contrast agents for molecular imaging with PET/MRI. Although beyond the scope of this proposal, my PET/MRI contrast agents have outstanding potential for clinical translation, which will provide a transformative ?game changing technology? for clinical PET/MRI.